It has previously been disclosed in U.S. Pat. No. 3,207,742, to Van de Castle, that hexamethylphosphoramide (HMPA) can be used in combination with lithium hydrocarbon catalysts to produce polymers from 1,3-dienes having an increased 1,2-microstructure. However, HMPA is a suspected carcinogenic and is therefore no longer desirable to be used in catalyst systems.
In a copending application to the same assignee, filed the same day as this application, by J. E. Hall and D. N. Schulz , there is disclosed a catalyst system employing substituted phosphoramide compounds for increasing 1,2-microstructure in polymers prepared from 1,3-dienes which are not suspected carcinogenic agents and reduce the rate of side reactions which occur during polymerization, thus adding stability to the polymerization reaction. The instant invention modifies the catalyst system of the copending application in order to increase the conversion of monomer to polymer and to produce a (co)polymer having an increased 1,2-microstructure in the 1,3-diene monomer contributed units.